Why did my creme fraiche turn out runny?

I mixed my creme fraiche (2 tbsp buttermilk, 1 cup heavy cream) in a mason jar and left it, unsealed, for 12 hours, then refrigerated it for four hours. It came out thick like sour cream, and was perfect. The second and third batches I made came out runny, but I used the same measurements, ingredients, and method. Why?